The present invention relates to a servo control device of a DC motor for controlling an object to be controlled of a servomechanism for controlling a mechanical position or an angle, for example, an exhaust gas recirculation (hereinafter referred to as EGR) valve provided in a recirculation system of an exhaust gas, a throttle valve, an actuator of a turbocharger and the like.
FIG. 1 shows a diagram in which a valve 11 of an EGR valve arranged in an exhaust gas return passage (c) for making an exhaust gas passage (a) of an engine E communicate with an intake gas passage (b) thereof is an object to be controlled and in which a DC motor 21 is driven by the output of a control section 50 to control the opening of the valve 11.
FIG. 2 is a longitudinal cross sectional view to show a constitution of the EGR valve to which a servo control device is applied having a DC motor 21 as a driving source. In FIG. 2, a reference numeral 1 denotes a valve body in which a passage communicating with the exhaust gas return passage (c) is disposed in a recirculation system of an exhaust gas. When the valve 11 is moved up as shown in the drawing, it is put into contact with a seat 12 to close the exhaust gas return passage c and when the valve 11 is moved down, it is separated from the seat 12 to open the exhaust gas return passage (c).
A reference numeral 21 denotes a DC motor to be a driving unit (torque generating source) of a valve for opening/closing an exhaust gas passage, 22 denotes a motor case mounted on the valve body 1, 23 denotes a rotor rotatably supported in the motor case 22 via bearings 24, 25, and the rotor 23 has a screw hole 23a made through the central portion of a shaft. A reference numeral 26 denotes a magnet mounted on the outer peripheral portion of the rotor 23, 27 denotes a stator core surrounding the outer periphery of the magnet 26, and a predetermined gap is formed between this stator core 27 and the above mentioned magnet 26.
A reference numeral 28 denotes a coil arranged in the stator core 27 and constituting a stator, 29 denotes a motor shaft to be a valve driving shaft, and this motor shaft 29 is made in a screw shaft and screwed into the screw hole 23a of the above mentioned rotor 23 and moved in an axial direction by the rotation of the above mentioned rotor 23.
A reference numeral 30 denotes a valve shaft abutting member integrally formed with the motor shaft 29. By making the top end of a valve shaft 13 having the above mentioned valve 11 at its top end abut against the tip (bottom end in FIG. 1) of this valve shaft abutting member 30, the valve shaft 13 is made to follow a movement in an axial direction of the above mentioned motor shaft 29 to thereby open/close the above mentioned valve 11.
A reference numeral 31 denotes a cover mounted on an end surface on the valve shaft abutting member 30 side in the motor case 22, 32 denotes a spring retaining member mounted on the tip side of the valve shaft abutting member 30, 33 denotes a spring hung between the spring retaining member 32 and the above mentioned cover 31 for urging the valve, and this spring 33 urges the valve 11 at the tip of the valve shaft via the valve shaft abutting member 30 in a direction that opens the valve 11. A reference numeral 34 denotes a commutator rotating integrally with the rotor 23 and having a shaft hole 34a at its central portion.
A reference numeral 35 denotes a power source side case mounted on an opening end on the commutator 34 side of the motor case 22. This power source side case 35 is an integrally molded part made of synthetic resin and constitutes a main body of a device for passing current through the DC motor 21 and is an integral combination of a built in sensor portion 36 in which a position sensor (not shown) for detecting the opening of the above mentioned valve is built and an input/output connector portion 37 in which a connector terminal is built.
A reference numeral 38 denotes a brush holding cylindrical portion integrally molded with a wall opposed to the commutator 34 in the power source side case 35, and 39 denotes a brush slidably inserted into the brush holding cylindrical portion 38 for passing a current. This brush 39 is made of carbon powder and copper powder and is connected via a lead wire (not shown) to the connector terminal of the above mentioned input/output connector portion 37.
Next, an operation will be described. Passing a current through the coil 28 via the brush 39 and the commutator 34 generates a rotational torque in the rotor 23 by an interaction between a magnetic field generated by the coil 28 and the magnetic field of the magnet 26. When the rotor 23 is rotated by the rotational torque, the motor shaft 29 screwed into the screw hole 23a of the rotor 23 is fed by the screw and moved straight in an axial direction.
Here, in a case where the motor shaft 29 is moved downward in FIG. 1, the valve shaft abutting member 30 is pushed by the motor shaft 29 in the direction of an urging force of the spring 33 to open the control valve 11 via the valve shaft 13 against which the valve shaft abutting member 30 abuts. Then, when the valve 11 moves near to a target position and a difference between a present position and a target position becomes nearly equal to zero, an electric power required to hold the valve 11 at its position against the restoring force of the return spring 18 is supplied to the DC motor 21.
On the other hand, when the rotor 23 of the DC motor 21 is rotated in a direction opposite to the above mentioned direction, the motor shaft 29 is moved upward in FIG. 1 and the valve shaft 13 abutting against the valve shaft abutting member 30 integral with the motor shaft 29 follows the motor shaft 29 by the restoring force of the return spring 18 to close the valve.
FIG. 3 is a diagram of one example of a circuit constitution to show a servo control device for controlling the driving of the above mentioned DC motor 21. In FIG. 3, a reference numeral 41 denotes a position sensor and this position sensor 41 has a movable contact 43 moving on a resistor 42 to which a constant voltage is applied by a power supply terminal and when the movable contact 43 moves with the rotation of the rotor 23, a voltage responsive to its moving position, that is to say, a voltage in compliance with the valve position is outputted as a detection signal.
A reference 44 denotes a controller block and this controller block 44 has an A/D input port 45 inputted with the detection signal from the valve position sensor 41, a filter 46 for reading the output of the A/D input port 45, a PI control computing section 48 for performing a PI control computation based on the output of the filter 46 and the output of a target value determining section 47, an excitation switching and duty computing section 49 for performing an excitation duty computation based on the output of the PI control computing section 48, a digital output port 50 for outputting an ON/OFF signal based on the output of the excitation switching and duty computing section 49 and a PWM output port 51 for outputting a PWM signal.
A reference numeral 52 denotes a switching circuit for controlling the passing of a current through the DC motor 21. A (+) side end of the DC motor 21 is connected to a connection point of semiconductor switching devices (hereinafter referred to as switch device) 53, 54 connected in series and a (xe2x88x92) side end thereof is connected to a connection point of semiconductor switching devices (hereinafter referred to as switch device) 55, 56 connected in series.
To each of the above mentioned switch devices 53, 55 is connected each of transistors 57, 58 which is turned ON by the output of the digital output port 50, and to each of the above mentioned switch devices 54, 56 is connected each of transistors 59, 60 which is turned ON/OFF by the output of the PWM output port 51.
Next, an operation will be described.
When a target value is determined by the target value determining section 47, the PI control computing section 48 performs the PI control computation based on the above mentioned target value and a detection value corresponding to a present valve position, which is detected by the position sensor 41 and inputted via the A/D input port 45 and the filter 46, and then the excitation switching and duty computing section 49 performs the excitation duty computation based on this computation result.
The digital output port 50 outputs an ON signal to a terminal (a) based on the output of the excitation switching and duty computing section 49 to bring the switch device 53 into conduction via the transistor 57 to thereby connect the (+) terminal of the DC motor 21 to a power applying terminal V. On the other hand, the PWM output port 51 outputs a PWM signal to an output terminal (b) based on the output of the excitation switching and duty computing section 49 to control a conduction of the switch device 56 via the transistor 60 to connect the (xe2x88x92) terminal of the DC motor 21 to an earth E thereby passing a current in a direction shown by a solid line.
In this manner, the DC motor 21 is operated to rotate the rotor 23, the motor shaft 29 is moved down by the rotation of its rotor 23, and by this moving down motion the valve shaft 13 is moved in the same direction against the return spring 18 to move the control valve 11 to a target position to thereby open the control valve 11. Then, when the control valve 11 moves near to the target position and the PI control computing section 48 recognizes that a difference between a detection value of a present valve position detected by the position sensor 41 and the target value becomes nearly equal to zero, the excitation switching and duty computing section 49 computes duty in response to the output signal of the PI control computing section 48 and supplies the DC motor 21 with an electric power required to hold the valve 11 at its target valve opening position against the restoring force of the return spring 18 based on this computed duty.
Since the conventional servomechanism to which a DC motor is applied is constituted as the above mentioned way, the valve is oscillated by the following causes when it performs the servo control near the target valve opening position.
(1) Since variations in torque are partially caused by variations in the current passing through the motor, the valve as a driven body to be subjected to a feed back control is not made stable near the target position.
(2) When the output of the sensor is read in, if the output of the sensor is mixed with motor noises, the output of the sensor can not be read in with high accuracy, so that the valve as the driven body to be controlled based on the output is not made stable in the vicinity of the target position.
There is presented a problem that the occurrence of oscillation of the valve prevents the valve from being stopped stably at the target position.
The present invention has been made to solve the above mentioned problem, and an object of the present invention is to provide a device for reducing the oscillation caused when a DC motor is subjected to a servo control.
A servo control device of a DC motor in accordance with the present invention includes: a DC motor for driving a driven body against a restoring force; a position sensor for detecting a moving position of the driven body; a speed detecting section for detecting a moving speed of the driven body based on a detection value of the position sensor; a deviation detecting section for detecting a deviation of the driven body based on the detection value of the position sensor; a PI control computing section that receives the detected moving speed and the detected deviation and a target value predetermined by a target value determining section and performs a PI control computation; a steady judgment section that judges from the moving speed and the deviation whether conditions of a steady region are held and starts the PI control computing section by an output signal when the condition of the steady region are judged to hold to thereby make the PI control computing section execute the PI control computation; and an excitation switching and duty computing section for outputting a power supply control signal of the DC motor based on an output signal of the PI control computing section.
By this arrangement it is made possible to reduce an oscillation of the valve in a case where the valve is held in a steady region that is vicinity of a target valve opening position.
In the servo control device of a DC motor in accordance with the present invention, the detection value from the position sensor is read in three times and the second largest detection value is adopted.
By this arrangement it is made possible to input the detection value of the position sensor with the steep noise removed and thus to correctly perform the servo control of the Dc motor even if noise caused by a brush part of the DC motor with a brush is introduced into the position sensor to make steep noise and this steep noise is inputted as the detection value of the position sensor.
A servo control device of a DC motor in accordance with the present invention includes: a DC motor for driving a driven body against a restoring force; a position sensor for detecting a moving position of the driven body; a PI control computing section that receives a detection value of the position sensor and a target value predetermined by a target value determining section and performs a PI control computation; an excitation switching and duty computing section for outputting a power supply control signal of the DC motor based on an output signal of the PI control computing section; a current detecting section for detecting current passing through the DC motor; and a control section for controlling the excitation switching and duty computing section so as to suppress a variation in the current passing through the DC motor based on a detection value of the current detecting section.
By this arrangement it is made possible to suppress a variation in torque caused by an abrupt variation in the current passing through the DC motor and thus to prevent the driven body from being oscillated.
In the servo control device of a DC motor in accordance with the present invention, the detection value from the position sensor is read in three times and the second largest detection value is adopted.
By this arrangement it is made possible to input the detection value of the position sensor with the steep noise removed and thus to correctly perform the servo control of the Dc motor even if noise caused by a brush part of the DC motor with a brush is introduced into the position sensor to make steep noise and this steep noise is inputted as the detection value of the position sensor.